The present invention relates to a scanning tunneling microscope for measuring the undulation on the surface of a sample by using a tunnel current and, more particularly, to a scanning tunneling microscope which is usable with a scanning electron microscope.
A scanning tunneling microscope whose resolution is as high as namometer order has recently been developed and is attracting increasing attention in various fields such as precision machining, superconduction, medicine and biology, not to speak of surface physics.
A scanning tunneling microscope is adapted to observe the undulation on the surface of a sample by using a tunnel current. When a probe made of tungsten or like metal polished at its tip to the order of 1 micrometer is brought close to the surface of a purified sample of, for example, silicon crystal to a distance of 1 nanometer order and, then, bias voltage of several millivolts to several volts is applied across the probe and the sample, a tunnel current is caused to flow between them. The tunnel current is greatly dependent upon the distance between the probe and the sample surface and is varied exponentially with the distance. Hence, if the probe is moved along the sample surface and, at the same time, the position of the probe is controlled such that the current is maintained constant (order of 10.sup.-10 to 10.sup.-7 amperes), any variation in the height of the sample surface can be determined by using the control signal. By moving the probe in an X and a Y direction, i.e., two-dimensionally along the sample surface, a three-dimensional image of the sample surface is produced.
A microscope of the kind described is capable of measuring undulation on a sample surface with accuracy which is 0.01 nanometer in the vertical direction and 0.2 to 0.3 nanometer in the horizontal direction, without destructing the sample.
A prerequisite with a scanning tunneling microscope is that its probe be moved extremely accurately in three different directions, i.e. X, Y and Z directions. Usually, such an accurate movement is implemented with a piezoelectric element actuator which comprises a laminate of several to several tens of piezoelectric ceramics that are connected in parallel. This piezoelectric element is deformable in proportion to a voltage which is applied from the outside, and it realizes control up to several micrometers by a relatively low voltage of about 100 volts.
However, a scanning tunneling microscope with a piezoelectric element type probe moving mechanism as described above has a drawback that its scanning area is limited. This prevents, for example, the structure of arrangement of steps of a monoatomic layer from being observed, although allowing the steps themselves to be observed. On the other hand, in the field of crystallography, how the step structure of a monoatomic layer is arranged is understood to have considerable significance.
Therefore, there is an increasing demand for the combined use of a scanning tunneling microscope and a scanning electron microscope. The combined use is such that the arrangement of steps is observed through an electron microscope and, thereafter, the step structure in the same position of the same sample is observed through a tunneling microscope. To meet this demand, a tunneling microscope has to be accommodated in the sample chamber of an electron microscope. However, accommodating a tunneling microscope in the sample chamber of an electron microscope has heretofore been impractical since the former has been designed for independent use and constructed in large dimensions to minimize the influence of vibrations and that of temperature drift and other thermal factors.
Further, the probe moving mechanism of a prior art scanning tunneling microscope generally includes three piezoelectric elements which are each assigned to a respective one of X, Y and Z directions. The piezoelectric elements are assembled in a tripod configuration, and a probe is fitted on the tip of the tripod to be controlled three-dimensionally in position. A problem with this arrangement in which three piezoelectric elements are simply combined is that, when one of them is contracted or expanded, the others are also deformed rendering the positional control inaccurate.
In the light of the above, there has been proposed a structure in which six X-direction piezoelectric elements and six Y-direction piezoelectric elements are combined in a lattice configuration, and a probe is mounted at their center through a Z-direction piezoelectric element. Then, the entire assembly is supported by four Z-direction piezoelectric elements. This kind of structure allows the probe to be moved in any of the X, Y and Z directions, facilitating position control. However, when such a probe moving mechanism is located in the sample chamber of an electron microscope, the numerous piezoelectric elements which are disposed above the probe constitute an obstruction to an electronic beam issuing toward a sample surface, which faces the tip of the probe or to secondary electrons coming out from the sample to be detected. Specifically, it is impossible to observe that area of a sample which is observed through a scanning tunneling microscope, to be observed through a scanning electron microscope.
When a tunnelling microscope is used to observe a sample, it is necessary that the distance between a probe and a sample surface be set beforehand to a one which allows a tunnel current to flow. Usually, such is accomplished by moving a stage on which a sample is laid. The stage is also moved when it is desired to change the observing position of a sample. To move the stage so, there has been used a mechanical means such as a feed screw mechanism or a cross roller guide mechanism. A mechanical means, however, cannot eliminate backlash, crosstalk and others which are detrimental to the accuracy of position control. Especially, in the case of a tunneling microscope as described above, position control of nanometer order is required which is almost impracticable with a mechanical means.
It has been customary, therefore, to move a stage of a tunneling microscope by a mechanical sample moving means and, then, to finely move a probe by use of a probe moving mechanism which is implemented with a piezoelectric element. Nevertheless, because the expanding and contracting stroke of a piezoelectric element is extremely short, it is very difficult to set a sample in an area in which the probe moving mechanism is capable of positioning. Moreover, when the positional relationship between the probe and the sample is determined by the probe moving mechanism as stated, the scanning region available is reduced by an amount corresponding to the movement of the probe. While the sample moving mechanism may be implemented with a piezoelectric element, simply causing the piezoelectric element to move the stage would fail to provide a sufficient stroke and, therefore, to accommodate samples of various thicknesses.